The acrosome reaction (AR) is a central event of mammalian fertilization involving the fusion and fenestration of sperm head membranes. The molecular events that initiate this exocytotic process are not completely understood. The goal of this research is to increase our knowledge of those events in human sperm. These studies will be carried out with an in vitro human sperm incubation system. My laboratory has found that progesterone, a steroid hormone present in follicular fluid and secreted by granulosa cells and cells of the cumulus oophorus, can initiate the human sperm AR by increasing sperm [Ca2+]. This is of particular interest because: 1) sperm have to pass through the cumulus extracellular matrix during in vivo human fertilization; 2) the [Ca2+] increase is so fast (< 1 min) that a sperm steroid surface receptor may be involved. These studies will include: 1) whether steroids increase [Ca2+] in the human sperm head; 2) determination of the steroid structure necessary for the increase in [Ca2+] and the AR; 3) experiments to help determine whether or not there is a plasma membrane steroid receptor; 4) investigation of the possible role of steroid inhibition of Na+, K+ -ATPase and [Ca2+ + Mg2+] -ATPase in increasing sperm head [Ca2+] and thereby the AR. Methods to be used include: in vitro incubation of human sperm, computerized fluorescence image analysis, transmission electron microscopy,immunofluorescent assay of the AR, HPLC, (45)Ca-efflux studies and ATPase assays with (32)P-substrate. It is hoped that these studies will result in a greater understanding of the molecular basis of human fertilization and thereby lead in the future to new methods of contraception and of treatment for infertility. In addition, the increasing of intracellular [Ca2+] by a steroid via a cell surface-mediated mechanism might have relevance to Ca2+-requiring events in somatic cells. Thus, these studies may also be of importance outside the field of fertilization.